Solifenacin succinate ((1S)-(3R)-1-azabicyclo[2,2,2]oct-3-yl-3,4-dihydro-1-phenyl-2(1H)-isoquinoline carboxylate succinate), represented by Formula (I) below, is a competitive and selective M3 muscarine receptor antagonist, and is known as a compound used to treat overactive bladder symptoms such as urgent urinary incontinence, urinary urgency, urinary frequency and the like.
[Formula I]

Conventional methods of preparing solifenacin or solifenacin succinate are disclosed in U.S. Pat. No. 6,017,927, International Patent Publication No. 2005/075474 (WO 2005/075474) and International Patent Publication No. 2005/105795 (WO 2005/105795).
U.S. Pat. No. 6,017,927 discloses two synthesis pathways for preparing solifenacin, synthesis pathway A and synthesis pathway B represented by Reaction Formula 1 below.
[Reaction Formula 1]

In the synthesis pathway A, the transesterification reaction of a racemic mixture of 1-phenyl-1,2,3,4-tetrahydroisoquinoline ethyl ester and quinuclidinol was conducted in a toluene suspension under the presence of sodium hydride (NaH), and the obtained mixture was refluxed and stirred. In this case, the obtained diastereomeric mixture was optically separated by high performance liquid chromatography (HPLC).
In the synthesis pathway B, quinuclidinyl chloroformate mono-hydrochloride was reacted with (1S)-1-phenyl-1,2,3,4-tetrahydroisoquinoline in the presence of sodium hydride (NaH) to synthesize solifenacin.
However, the method of preparing solifenacin using the synthesis pathway A and synthesis pathway B is problematic in that it requires high cost, its efficiency is not high due to post-treatment processes, and it is not suitable for producing solifenacin on an industrial scale.
Moreover, ethyl carboxylate used in the synthesis pathway A produces ethanol as a by-product of a transesterification reaction. In this case, since ethanol initiates a nucleophilic attack against solifenacin in the presence of a base, in order to continue the reaction, there is a problem in that ethanol must be separated from a reaction system using an azeotrope with toluene or a method related thereto, which is very industrially difficult. Further, the synthesis pathway A is also problematic in that it is difficult to obtain solifenacin having high optical purity because solifenacin is racemized.
Further, in the synthesis pathway A and the synthesis pathway B, the mixture was heated under reflux in order to accelerate the reaction in the solifenacin synthesis process, and a very strong base, such as sodium hydride (NaH), was used. Therefore, there are problems in that it is not easy to control the reaction and it is difficult to produce solifenacin on an industrial scale.
International Patent Publication No. 2005/075474 (WO 2005/075474) discloses another synthesis pathway for preparing solifenacin and solifenacin succinate, as represented by Reaction Formula 2 below.

However, as mentioned in U.S. Pat. No. 6,017,927, this synthesis pathway of Reaction Formula 2 is also problematic in that a reaction is performed using ethylchloroformate in the presence of a base, and thus ethanol is produced in the second step of the reaction as a by-product.
International Patent Publication No. 2005/105795 (WO 2005/105795) discloses another synthesis pathway for preparing solifenacin, as represented by Reaction Formula 3 below.

As shown in Reaction Formula 3 above, solifenacin is prepared through an intermediate, which is formed from (1S)-1-phenyl-1,2,3,4-tetrahydroisoquinoline with a leaving group(Lv) such as 1H-imidazole-1-yl, 2,5-dioxopyrrolidine-1-yloxy, 3-methyl-1H-imidazol-3-lium-1-yl or chloride and then conducted a condensation reaction with (R)-quinuclidinol in a mixed solvent of toluene and dimethylsulfoxide or in a single solvent of toluene by refluxing and stirring in the presence of sodium hydride (NaH).
However, this synthesis pathway of Reaction Formula 3 is also problematic in that it is not easy to control a reaction process because a strong base such as sodium hydride (NaH) is used, a purification process using chromatography is required and a moisture-sensitive leaving group is used.
Accordingly, in order to solve the above-mentioned problems, the present inventors have devised a high yield method of preparing solifenacin or a salt thereof, in which high-purity solifenacin or a salt thereof can be simply and efficiently prepared at room temperature without using a base, and which can be industrially used.